Separation of ethanol from isopropanol by azeotropic distillation

ABSTRACT

Ethanol is difficult to separate from isopropanol by conventional distillation or rectification because of the proximity of their boiling points. Ethanol can be readily separated from isopropanol by azeotropic distillation. Effective agents are sec. butyl acetate, hexene-1 and 1,3-dioxolane.

FIELD OF THE INVENTION

This invention relates to a method for separating ethanol from isopropanol using certain organic liquids as the agent in azeotropic distillation.

DESCRIPTION OF PRIOR ART

Azeotropic distillation is the method of separating close boiling compounds or azeotropes from each other by carrying out the distillation in a multiplate rectification column in the presence of an added liquid, said liquid forming an azeotrope with one or both of the compounds to be separated. Its presence on each plate of the rectification column alters the relative volatility in a direction to make the separation on each plate greater and thus require either fewer plates to effect the same separation or make possible a greater degree of separation with the same number of plates. The azeotrope forming agent is introduced with the feed to a continuous column. The azeotrope forming agent and the more volatile component are taken off as overhead product and the less volatile component comes off as bottoms product. The usual methods of separating the azeotrope former from the more volatile component are cooling and phase separation or solvent extraction.

The usual method of evaluating the effectiveness of azeotropic distillation agents is the change in relative volatility of the compounds to be separated. Table 1 shows the degree of separation or purity obtainable by theoretical plates at several relative volatilities. Table 1 shows that a relative volatility of at least 1.2 is required to get an effective separation by rectification.

                  TABLE 1                                                          ______________________________________                                         Effect of Relative Volatility on Theoretical Stage Requirements.               Separation Purity,                                                                        Relative Volatility                                                 Both Products                                                                             1.02    1.1    1.2  1.3 1.4  1.5  2.0 3.0                           (Mole Fraction)                                                                           Theoretical Stages at Total Reflux                                  ______________________________________                                         0.999      697     144    75   52  40   33   19  12                            0.995      534     110    57   39  30   25   14  9                             0.990      463     95     49   34  26   22   12  7                             0.98       392     81     42   29  22   18   10  6                             0.95       296     61     31   21  16   14   8   4                             0.90       221     45     23   16  12   10   5   3                             ______________________________________                                    

There are a number of commercial processes which produce complex mixtures of alcohols, e.g. the Fischer-Tropsch process which produces a series of homologous alcohols. Two of the commonest alcohols usually present are ethanol, B.P.=78° C. and isopropanol, B.P.=82° C. The relative volatility between these two is 1.18 which makes it very difficult to separate them by conventional rectification. Azeotropic distillation would be an attractive method of effecting the separation of ethanol from isopropanol if agents can be found that (1) will create a large apparent relative volatility between ethanol and isopropanol and (2) are easy to recover from the ethanol. Table 2 shows the relative volatility required to obtain 99% purity. With no agent, the relative volatility is 1.18 and 75 actual plates are required. With an agent giving a relative volatility of 2.0, only 16 plates are required.

                  TABLE 2                                                          ______________________________________                                         Theoretical and Actual Plates Required vs. Relative                            Volatility for 1-butanol-2-Pentanol Separation                                 Relative                                                                              Theoretical Plates Required                                                                      Actual Plates Required                                Volatility                                                                            At Total Reflux, 99% Purity                                                                      75% Efficiency                                        ______________________________________                                         1.18   56                75                                                    1.6    20                27                                                    2.0    12                16                                                    ______________________________________                                    

OBJECTIVE OF THE INVENTION

The object of this invention is to provide a process or method of azeotropic distillation that will enhance the relative volatility of ethanol from isopropanol in their separation in a rectification column. It is a further object of this invention to identify organic compounds which in addition to the above constraints, are stable, can be separated from ethanol and recycled to the azeotrope column with little decomposition.

SUMMARY OF THE INVENTION

The objects of this invention are to provide a process for separating ethanol from isopropanol which entails the use of certain organic compounds as the agent in azeotropic distillation.

DETAILED DESCRIPTION OF THE INVENTION

I have discovered that certain organic compounds will greatly improve the relative volatility of ethanol to isopropanol and permit the separation of ethanol from isopropanol by rectification when employed as the agent in azeotropic distillation. Table 3 lists the compounds that I have found to be effective. They are methyl formate, t-butyl methyl ether, isopropyl ether, methyl isopropyl ketone, 2,2-dimethoxypropane, ethyl formate, t-amyl methyl ether, methyl propionate, propyl formate, tetrahydrofuran, acetone, methyl ethyl ketone, hexane, hexene-1, cyclopentane, cyclohexane, 2,4-dimethyl pentane, cyclohexanone, methyl cyclohexane, 1-heptene, 1-octene, 1,3-dioxolane, benzene, toluene, acetal, methyl acetate, ethyl propionate, 2-pentanone, dimethoxymethane, isopropyl acetate, sec. butyl acetate, t-butyl acetate and nitromethane.

                  TABLE 3                                                          ______________________________________                                         Effective Azeotropic Distillation Agents For                                   Separating Ethanol From Isopropanol                                                               Relative                                                    Compounds          Volatility                                                  ______________________________________                                         None               1.18                                                        Methyl formate     1.25                                                        t-Butyl methyl ether                                                                              1.25                                                        Isopropyl ether    1.35                                                        Methyl isopropyl ketone                                                                           1.3                                                         2,2-Dimethoxypropane                                                                              1.25                                                        Ethyl formate      1.35                                                        t-Amyl methyl ether                                                                               1.6                                                         Methyl propionate  1.35                                                        Propyl formate     1.25                                                        Tetrahydrofuran    2.0                                                         Acetone            2.0                                                         Methyl ethyl ketone                                                                               2.1*                                                        Hexane             1.4                                                         Cyclopentane       1.22*                                                       Cyclohexane        1.35                                                        Hexene-1           1.72*                                                       2,4-Dimethyl pentane                                                                              1.45                                                        Cyclohexanone      1.58*                                                       Methyl cyclohexane 1.35                                                        1-Heptene          1.25                                                        1-Octene           1.7                                                         1,3-Dioxolane      1.97*                                                       Benzene            1.35                                                        Toluene            1.4                                                         Acetal             1.25                                                        Methyl acetate     1.25                                                        Ethyl propionate   1.35                                                        t-Butyl acetate    1.55                                                        2-Pentanone        1.3                                                         Dimethoxymethane   1.45                                                        Isopropyl acetate  1.3                                                         sec. Butyl acetate 1.45                                                        Nitromethane       1.45                                                        ______________________________________                                          *Data Obtained in a Multiplate Column                                    

THE USEFULNESS OF THE INVENTION

The usefulness or utility of this invention can be demonstrated by referring to the data presented in Tables 2 and 3. All of the successful agents show that ethanol can be separated from isopropanol by means of azeotropic distillation in a rectification column and that the ease of separation as measured by relative volatility is considerable.

WORKING EXAMPLES Example 1

Twenty-seven grams of isopropanol, 13 grams of ethanol and 40 grams of sec. butyl acetate were charged to a vapor-liquid equilibrium still and refluxed for seven hours. Analysis indicated a vapor composition of 51.6% isopropanol, 48.4% ethanol; a liquid composition of 60.9% isopropanol, 39.1% ethanol. This is a relative volatility of 1.45.

Example 2

Sixty grams of ethanol, 40 grams of isopropanol and 150 grams of hexene-1 were placed in the stillpot of a 5.6 theoretical plate glass perforated plate rectification column and refluxed for five hours. The overhead composition was 97% ethanol, 3% isopropanol; the bottoms composition was 61.4% ethanol, 38.6% isopropanol. This is a relative volatility of 1.72.

Example 3

Fifty grams of ethanol, 50 grams of isopropanol and 140 grams of 1,3-dioxolane were placed in the still pot of a 5.6 theoretical plate glass perforated plate rectification column and refluxed for five hours. The overhead composition was 97.8% ethanol, 2.2% isopropanol; the bottoms composition was 50% ethanol, 50% isopropanol. This is a relative volatility of 1.97. 

I claim:
 1. A method for recovering ethanol from a mixture of ethanol and isopropanol which comprises distilling a mixture of ethanol and isopropanol in the presence of an azeotrope forming agent, recovering the ethanol and the azeotrope forming agent as overhead product and obtaining the isopropanol as bottoms product, wherein said azeotrope forming agent consists of one material selected from the group consisting of methyl formate, t-butyl methyl ether, isopropyl ether, 2,2-dimethoxypropane, ethyl formate, t-amyl methyl ether, tetrahydrofuran, acetone, cyclopentane, hexene-1, cyclohexanone, 1-heptene, 1-octene, 1,3-dioxolane, acetal, dimethoxymethane, and nitromethane. 